Biodiesel is produced from animal or vegetable fats, oils or split fatty acids. The raw material currently preferred for biodiesel production in Europe is rapeseed oil. The European patent specification EP 0523767 B1, for instance, describes a process for producing fatty acid methyl ester or fatty acid ethyl ester and glycerol by transesterification of oils or fats, wherein rapeseed oil here is used as raw material. After an optional pretreatment with methanol or ethanol and an alkaline catalyst in liquid phase, the oil or fat is converted to the products fatty acid methyl ester or fatty acid ethyl ester, and glycerol as a byproduct. Advantageously, at least two reaction stages are employed, wherein each reaction stage includes a mixing reactor and a separator for separating a light phase rich in fatty acid alkyl ester and a heavy phase rich in glycerol. The light phase rich in fatty acid alkyl ester is supplied to further processing, which among other things includes washing with water in a washing column.
In North America, large amounts of soybean oil are also used for biodiesel production. In Asia, palm oil or palm olein are used as starting material for the production of biodiesel. All vegetable oils contain significant concentrations of steryl glycosides in dissolved form. These are widely used vegetable ingredients, which from a chemical point of view result from the combination of a carbohydrate (e.g. glucose) with a phytosterol and are also known as sterolins. Phytosterols are vegetable hormones and are constituents of vegetable cell membranes; known representatives include e.g. stigmasterol or campesterol.
The concentration of steryl glycosides in the vegetable oils can vary strongly depending on kind, origin and pretreatment. By way of example, raw soybean oil can contain up to 2300 ppm and palm oil up to 3000 ppm of steryl glycosides, whereas a type of oil poorer in steryl glycosides, such as sunflower oil, only contains about 300 ppm. Rapeseed oil typically contains up to 500 ppm of steryl glycosides, but the content greatly depends on the pretreatment of the oil.
The steryl glycosides also can carry a fatty acid radical, which is bound to a hydroxyl group of the carbohydrate by esterification; this is referred to as acylated steryl glycosides. In the production of biodiesel, the same partly are converted to non-acylated steryl glycosides, whereby their solubility in organic solvents is reduced.
The use of oil types rich in steryl glycosides in the production of biodiesel can lead to the formation of turbidities or precipitates during the subsequent storage of the product at cold ambient temperatures. Since in a conventional washing column, as it is used for instance in the production of biodiesel from rapeseed oil, steryl glycosides cannot be removed due to their poor solubility in water, obstructions may occur there as a result of the formation of precipitates rich in steryl glycosides. In the succeeding process steps, the steryl glycosides can lead to deposits in the heat exchangers of the driers and clog filters. The biodiesel produced is not on specification, since the specified value for overall pollution is not complied with. Processing such oils therefore requires an increased maintenance effort for the production plant. Furthermore, as a result of the necessary shut-down of the production plant or its parts for maintenance purposes, the availability and hence productivity of the production plant is restricted. The precipitates can be removed by mechanical methods such as filtration or centrifugation, possibly also in combination.
The addition of an adsorbent or filtration aid for separating steryl glycosides from biodiesel and for the subsequent removal thereof by means of filtration, centrifugation or combinations thereof is described in the patent specification WO 2007/076163 A2. It is disadvantageous here that this treatment only is possible in a simple way for the finished biodiesel product. What is furthermore disadvantageous is the use of large amounts of solid filtration or adsorption aids and the resulting production of large amounts of solid wastes. As a further option, the distillative purification of biodiesel is mentioned in this patent specification, since the steryl glycosides have a considerably higher boiling point than fatty acid alkyl esters. However, this option involves a higher expenditure of equipment and a very high expenditure of energy, since the entire fatty acid alkyl ester must be removed by distillation as a low boiler.